The long term objectives of this work is to develop a rapid, colorimetric instrument independent assay for cardio digitalis constituents, notably digoxin and digitoxin. Genesis Labs' "IMMUNOSTRIP" assay (protected by patent) is constructed in a configuration, which is applicable for the rapid detection of drugs and metabolites in biological fluids e.g. urine, blood or saliva. The relative simplicity of the method suggests its easy use by medically unsophisticated individuals. We will attempt initially, to determine digoxin in serum at a level of 5 ng/ml. Ultimately, our aim is to increase the sensitivity of the assay system into the 1-5 ng/ml range. This will require the use of a hand held reflectometer ($1000 per unit currently) for reading the strips, however it is feasible by varying the amount of antigen and enzyme modified anti-digoxin antibody within the IMMUNOSTRIP that we can produce a visual assay within this lower range. The assay has many applications for monitoring therapeutic drug including digoxin, and therefore will have a major impact on our knowledge of the health benefit/risk ratio within populations of our society who require drugs. Based on our ability to monitor other hormones, drugs and small haptens using this assay, we feel confident that in the future by using highly specific polyclonal antisera or more likely monoclonal antibodies, it may be possible to monitor active digoxin and thereby predict therapeutic versus toxic levels of digoxin in a physician's office setting with a one minute test.